This invention relates generally to propylene polymers, to methods for their production and to methods for their use in oriented and non-oriented film applications. More particularly, the present invention relates to a two-stage polymerization process used to make tailored crystallinity resins and random copolymers of propylene and ethylene using a dual metallocene catalyst system.
Polypropylene films are widely used in tape applications and in packaging applications such as food packaging. The optimization of processing characteristics and film properties of propylene based films has been the subject of intense effort. U.S. Pat. No. 5,118,566, for example, describes a biaxially oriented film made from polypropylene, a natural or synthetic resin, and a nucleating agent. The process for producing this film includes biaxially drawing the film at temperatures below the melting point of the polypropylene.
Critical to the rheological properties of the film is the starting propylene polymer. For instance, as discussed by A. K. Mehta et al. in 2 METALLOCENE-BASED POLYOLEFNS 463-488 (J. Scheirs and W. Kaminsky eds., John Wiley and Sons 2000), properties such as the molecular weight distribution (MWD), composition distribution (CD), tacticity, and xcex1-olefin incorporation are important in final film performance. Attempts to achieve these optimal properties have been made with polymerization processes catalyzed by conventional Ziegler-Natta (ZN) catalysts as well as single metallocene catalysts. While metallocene catalyzed propylene polymer is known, single metallocene, single stage production processes are often inadequate to achieve the range of desired properties for films. To achieve ideal performance, polypropylene production methods must be tailored to meet the desired characteristics. One such production method that can tailor the resultant polypropylene polymer is a multi-stage metallocene process, where the xcex1-olefin incorporation can take place in one or more stages during the propylene polymerization.
Multiple stage polymerization processes are known in the art as is the use of multiple metallocenes. However, multiple stage polymerization processes are typically used to prepare block copolymers which contain rubbery materials as opposed to the crystalline polymers. U.S. Pat. No. 5,280,074; 5,322,902, 5,346,925, and WO 98/10016, for example, describe two-stage processes for producing propylene block copolymers. The propylene/ethylene copolymer portion of these compositions is a non-crystalline, rubbery material suitable for molding applications rather than films. U.S. Pat. No. 5,708,090 discuses a two stage process for making polypropylene based materials for high impact molding applications. EP 0 704 463 discloses a two stage process using metallocenes that produce polymer suitable for low impact resistance applications having a relatively high comonomer content.
Ziegler-Natta based propylene polymers and copolymers have been used to make films as described in U.S. Pat. No. 5,298,561. U.S. Pat. No. 5,350,817 and Canadian Patent Application No. 2,133,181 describe the use of two or more metallocenes for the preparation of isotactic propylene polymers, however, neither reference describes a multiple stage process for the production of crystalline polymers such as those described herein.
There is a need to produce propylene homopolymer and copolymer films having a combination of desirable attributes that is not achieved in the prior art. A film is needed that combines the attributes of high melt strength at low shear rate while improving shear thinning at high shear rate. A film should also have a broad processing window during fabrication and a favorable balance of film properties not yet achieved in the art.
The present invention is directed towards such a film, and the propylene polymers used to make such a film, wherein the polymer resulting from the method of the invention has a broadened molecular weight distribution in at least the second stage of the process, while maintaining a high degree of crystallinity.
The inventors of the present invention have found that the properties of propylene polymers can be tailored by forming either a tailored crystallinity resin, or a random copolymer. A xe2x80x9ctailored crystallinity resinxe2x80x9d (TCR) is a polymer made by polymerizing propylene in one stage and then further polymerizing propylene and a minor amount of comonomer in a separate stage using a metallocene catalyst system comprising at least two metallocenes in each of the stages. A xe2x80x9crandom copolymerxe2x80x9d (RCP), as used here, is a crystalline propylene composition made by polymerizing propylene and a minor amount of comonomer in a single stage or multiple stage process using a metallocene catalyst system comprising at least two metallocenes in each of the stages, wherein the comonomer is incorporated into all stages of the polymerization. The resulting polymers of this invention have surprisingly high molecular weights and broad molecular weight distributions, and offer processability benefits in oriented and non-oriented film applications. Films made from these unique polymers have a significantly broader processability range and can be evenly stretched at lower temperatures compared to the polypropylene films available today. The resulting films have a favorable balance of properties including high strength, good optical properties and good barrier properties.
Specifically, the inventors have found that the incorporation of less than or equal to 1 wt % ethylene comonomer in one embodiment, and from 1 to 5 wt % ethylene comonomer in another embodiment, during the polymerization process as described herein results in a substantial broadening of the molecular weight distribution at the high molecular weight end as evidenced by the relatively large value in the molecular weights, and Mz values. This is unexpected since with both the individual metallocenes used alone the molecular weight drops with ethylene addition. The molecular weight broadening improves the Theological properties of the film as described herein.
The objects of the invention are achieved by providing a propylene homopolymer and copolymer having a broad molecular weight distribution, while maintaining a high level of crystallinity. This is achieved by providing in one embodiment a TCR propylene polymer comprising: (a) from 10 to 90 wt % homopolymerized crystalline propylene units; and (b) from 90 to 10 wt % crystalline propylene copolymer wherein the wt % of the comonomer based on the total weight of the polymer is in the range of from 0.05 to 15; wherein each polymer is prepared in a separate stage utilizing in each stage a metallocene catalyst system comprising two metallocene catalyst components and wherein the polymer has a molecular weight distribution (Mw/Mn) in the range of from 2.1 to 10. In another embodiment, the objectives are achieved by providing an RCP crystalline propylene copolymer, wherein the wt % of the comonomer based on the total weight of the polymer is in the range from about 0.05 to about 15, wherein the RCP is prepared in a single stage or multiple stage utilizing in each stage a metallocene catalyst system comprising two metallocene catalyst components, and wherein the RCP has a molecular weight distribution (Mw/Mn) in the range from about 2.1 to about 10.